Effective operation of certain semiconductor radiation detectors, such as PIN diode (p-type—intrinsic—n-type) detectors and SDDs (silicon drift detectors), benefits from sealing the detector crystal and its immediate electric contacts in a gas-tight enclosure known as the detector can. In some applications the semiconductor radiation detector may be located at the end of an elongated pipe, which points towards the sample holder of a micro-analyzer. A radiation window at the front face of the detector can or pipe allows radiation to enter, while the back side of the detector assembly has the attachment means and contact pins that are needed to couple the detector mechanically, electrically, and thermally to the radiation-detecting appliance. A thermoelectric cooler, such as a Peltier element, is typically included in the entity, which in assembled form is frequently referred to as the detector head.
FIG. 1 illustrates a prior art detector head. The detector chip 101 is attached to a substrate 102, which in turn is attached to a thermoelectric cooler 103. A base plate 104 with a protruding attachment bolt 105 supports the arrangement and closes the bottom of the detector can 106, the front face of which has an opening covered by a radiation window 107. The graphical illustration is simplified and omits possible intermediate shielding layers and other features that are not essential for understanding the background of the present invention.
Contact pins 108 go through holes in the base plate 104, and are electrically isolated therefrom by insulator sleeves 109. The entity that consists of the base plate 104, the attachment bolt 105, the contact pins 108, and the insulator sleeves 109 is sometimes called the header. A bonding wire 110 connects the top end of each contact pin 108 to a bonding pad 111 on the top surface of the substrate 102. Further bonding wires (not shown) may constitute the electric connections between areas of the substrate 102 and respective contact pads on the detector chip 101. The choice of just wire bonding as the technology of making the electric connections involves the inherent advantage of low thermal conductivity; for example, it is essential that as little heat as possible flows from the contact pins 108 to the substrate 102 and further to the detector chip 101.
The sensitivity of the detector is in principle better with larger detector chips: the larger surface of the detector chip collects more photons, increasing the pulse frequency. In the arrangement of FIG. 1 the substrate 102, and thus also the detector chip 101, must be smaller in diameter than the circle formed by the contact pins 108, because the bonding wedge must be able to touch the top ends of the contact pins, as well as the bonding pads on the substrate 102 and detector chip 101, from above. Basically it would be possible to make the whole detector head larger in diameter, but that would make it more difficult to place it close to the sample from which originates the radiation to be detected. The diameter of the ring of contact pins must always be significantly smaller than the diameter of the base plate, because hermetically sealing and electrically insulating the contact pin holes in the base plate necessitate the use of an insulator sleeve that completely encircles the contact pin and has a certain minimum wall thickness, and because some base plate metal must remain between the holes for the insulator sleeves and the base plate edge.
FIGS. 2 and 3 illustrate known detector heads that aim at avoiding the size-limiting effect of the circle of contact pins. The structure of FIG. 2 is known from the patent publication EP 2286275. The bonding is made from the side, so the bonding wire 210 connects the side surface of the contact pin 208 to a bonding pad 211 located on a vertical surface of the substrate 202. In this arrangement the contact pins are shorter than in the arrangement of FIG. 1, and the upper limit for the diameter of the substrate 202 (and consequently, to almost the same extent, for the diameter of the detector chip 201) is determined by the diameter of the circle of contact pins plus the marginal difference allowed by the bonding. In practice one may say that it is the diameter of the circle of contact pins that defines the maximum diameter of the substrate.
The structure of FIG. 3 is known from the patent publication US 2012/0228498. The detector chip 301 is flip-chip-bonded to a first substrate 302, under which is a second substrate 312, the outer edge of which defines cavities. For connections to and from the contact pins, bonding wires 310 go between the top ends of the contact pins 308 and bonding pads (not separately shown) located in said cavities. Bonding to the top side of the detector chip 301 is typically also needed, although not shown in FIG. 3. The diameter of the second substrate 312 must be smaller than that of the circle of contact pins, but the diameter of the first substrate 302 and the detector chip 301 can be of the same size or even larger than the diameter of the circle of contact pins. Disadvantages of this structure include the relatively complicated three-dimensional form of the second substrate, as well as the small and three-dimensionally limited space available in the cavities for making the wire bonding.
A need exists for detector head structures that would enable the detector chip to have a large area while simultaneously making the structure easy to manufacture and reliable in use.